Prediction of the stability of the rhombohedral phase in IV-VI monochalcogenides and its origin

The rhombohedral monotellurides, GeTe and SnTe, are non-centrosymmetric materials with ferroelectric behavior and potential applications in thermoelectricity and spintronics. In a previous computational study [E. Segev, U. Argaman, R. E. Abutbul, Y. Golan and G. Makov, CrystEngComm, 2017, 19, 1751-1761], it was found that the rhombohedral phase may also be the thermodynamically most stable phase in SnSe and GeSe at low temperatures. In the present study, we explore the mechanical and thermodynamic stability of the rhombohedral phase in these systems and its enhancement as a function of pressure and temperature using density functional theory calculations. Within the region of stability, we examine the structure, bonding and physical properties of the rhombohedral phase. This phase is a distorted rock-salt phase forming a non-spherical lone-pair with an opposite directionality to the chemical bond. The monochalcogenides in this phase exhibit ferroelectric behavior, and we calculate the electric polarization. Finally, the transition from a rhombohedral to rock-salt phase at high pressures is examined and found to be second-order at zero Kelvin.